The Battle of Eylau or Battle of Preussisch-Eylau, 7 and 8 February 1807, was a bloody and strategically inconclusive battle between Napoleon's Grande Arme and the Imperial Russian Army under the command of Levin August von Bennigsen near the town of Preussisch Eylau in East Prussia. Late in the battle, the Russians received timely reinforcements from a Prussian division of von L'Estocq. After 1945 the town was renamed Bagrationovsk as a part of Kaliningrad Oblast, Russia. The engagement was fought during the War of the Fourth Coalition, part of the Napoleonic Wars.

Napoleon's armies previously smashed the army of the Austrian Empire in the Ulm Campaign and the combined Austrian and Russian armies at the Battle of Austerlitz on 2 December 1805. On 14 October 1806 Napoleon crushed the armies of the Kingdom of Prussia at the Battle of JenaAuerstedt and hunted down the scattered Prussians at Prenzlau, Lbeck, Erfurt, Pasewalk, Stettin, Magdeburg and Hamelin.

In late January Bennigsen's Russian army went on the offensive in East Prussia, pushing far to the west. Napoleon reacted by mounting a counteroffensive to the north, hoping to prevent their retreat to the east. After his Cossacks captured a copy of Napoleon's orders, Bennigsen rapidly withdrew to the northeast to avoid being cut off. The French pursued for several days and found the Russians drawn up for battle at Eylau.

In a vicious evening clash the French captured the village, with heavy losses on both sides. The following day brought even more serious fighting. Early in the battle a frontal attack by Napoleon failed, with catastrophic losses. To reverse the situation, the emperor launched a massed cavalry charge against the Russians. This bought enough time for the French right wing to throw its weight into the contest. Soon the Russian left wing was bent back at an acute angle and Bennigsen's army was in danger of collapse. A Prussian corps belatedly arrived and saved the day by pushing back the French right. As darkness fell, a French corps tardily appeared on the French left. That night Bennigsen decided to retreat, leaving Napoleon in possession of a snowy battlefield covered with thousands of dead and wounded. Eylau was the first serious check to the Grande Arme, and the myth of Napoleon's invincibility was badly shaken. However, the French would go on to win the war by decisively defeating the Russians on 14 June at the Battle of Friedland.

Levin August Gottlieb Theophil Graf von Bennigsen (Russian: Ле́вин А́вгуст Го́тлиб Теофи́ль фон Бе́ннигсен, romanized: Lévin Ávgust Gótlib Teofíl' fon Bénnigsen, also Лео́нтий Лео́нтьевич Бе́ннигсен (Leóntii Leónt'yevich); 10 February 1745 – 3 December 1826) was a German general in the service of the Russian Empire.